In history class, the girl to boy ratio is $7$ to $4$. If there are $35$ girls, how many boys are there?
A ratio of $7$ girls to $4$ boys means that there is a group of $7$ girls for every group of $4$ boys. If there are $35$ girls, then there are $5$ groups of $7$ girls each. Because we know the ratio is $7:4$ , there must be $5$ groups of $4$ boys each. There is a total of $20$ boys in history class.